1. Field of the Invention
The present invention relates to optical communications systems and, in particular, to a wide band optical amplifier.
2. Description of the Related Art
In general, fiber amplifiers are crucial elements for high-performance fiber-optic communications systems and networks. The ability to directly amplify optical signals, without converting them to some other form for processing, is highly desirable for communications. Various designs of silica-based earth-doped optical fibers, i.e., erbium-doped fiber amplifiers (EDFAs), have been employed to amplify optical signals in communication systems. Increasing the EDFA gain bandwidth increases the system capacity. Complex techniques are used to achieve a wide gain in the conventional wavelength range between 1530 nm-1560 nm (hereinafter referred to as xe2x80x9cC-bandxe2x80x9d) and the long wavelength range between 1570 nm-1610 nm (hereinafter referred to as xe2x80x9cL-bandxe2x80x9d) for the wavelength-division-multiplexed (WDM) transmission system.
FIG. 1 illustrates a schematic view of conventional broadband EDFA, which are capable of producing a broad optical bandwidth. Basically, the conventional EDFA is divided into two amplification sections. The input signals are split into two sub-bands, the C-band and the L-band are amplified independently. Then, the amplified signals are recombined afterwards to produce an output signal. FIG. 2 illustrates the output power spectrum and the noise figure spectra of the prior art system of FIG. 1. Here, the gain is produced by an erbium-doped silica fiber.
As shown in FIG. 1, the L-band fiber amplifier typically requires a much longer length of EDFs (188 m) and more power pumps as the power inversion has typically been lower than that observed in C-band EDFAs. Thus, the installation of the prior art system is not cost-effective due to longer EDFAs and higher pump power requirements. Accordingly, there is a need for a cost-effective optical power amplifier with improved power conversion efficiency.
The present invention is directed to an optical amplifier with a structure that efficiently utilizes erbium-doped fiber amplifiers (EDFAs) and the associated pump source, and as a consequence provides an increased signal bandwidth using much shorter erbium-doped fibers (EDFs) and less light pump power.
Accordingly, the inventive optical fiber includes a first erbium-doped fiber amplification stage being pumped by a first pump light source; a second erbium-doped fiber amplifier stage being pumped by a second pump light source; a split section disposed between the first and second amplification stages for splitting the amplified signal light into a first sub-band and a second sub-band, a reflector for reflecting the amplified output of the second amplifier stage back into the second amplifier stage in a reverse direction; a combiner for combining the reflected output, in succession, from the second amplifier and first amplifier to produce an output signal; and, a circulator for redirecting the reflected output traveling in a reverse direction to the input of the combiner.
The method of amplifying optical signals having two or more optical bands includes the steps of: passing input optical signals through a first amplifier stage; splitting the amplified input signals into a C-band and L-band signals; further amplifying the L-band signal in a second amplifier stage; redirecting the amplified L-band again back into the second amplifier stage in a reverse direction; and, recombining the redirected L-band and the C-band to produce an output signal.
The foregoing and other features and advantages of the invention will be apparent from the following, more detailed description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, the emphasis instead is placed upon illustrating the principles of the invention.